The getsockopt() and
setsockopt() system calls manipulate the
options associated with a socket. Options may exist
at multiple protocol levels; they are always present at the
uppermost ’’socket’’ level.

When
manipulating socket options the level at which the option
resides and the name of the option must be specified. To
manipulate options at the socket level, level is
specified as SOL_SOCKET. To manipulate options at any other
level the protocol number of the appropriate protocol
controlling the option is supplied. For example, to indicate
that an option is to be interpreted by the TCP protocol,
level should be set to the protocol number of TCP;
see getprotoent(3).

The
optval and optlen arguments are used to access
option values for setsockopt(). For
getsockopt() they identify a buffer in which the
value for the requested option(s) are to be returned. For
getsockopt(), optlen is a value-result
argument, initially containing the size of the buffer
pointed to by optval, and modified on return to
indicate the actual size of the value returned. If no option
value is to be supplied or returned, optval may be
NULL.

The
optname argument and any specified options are passed
uninterpreted to the appropriate protocol module for
interpretation. The include file <sys/socket.h>
contains definitions for socket level options, described
below. Options at other protocol levels vary in format and
name; consult the appropriate entries in section 4 of the
manual.

Most
socket-level options utilize an int argument for
optval. For setsockopt(), the argument should
be non-zero to enable a boolean option, or zero if the
option is to be disabled. SO_LINGER uses a struct
linger argument, defined in <sys/socket.h>,
which specifies the desired state of the option and the
linger interval (see below). SO_SNDTIMEO and SO_RCVTIMEO use
a struct timeval argument, defined in
<sys/time.h>.

The following
options are recognized at the socket level. For
protocol-specific options, see protocol manual pages, e.g.
ip(4) or tcp(4). Except as noted, each may be examined with
getsockopt() and set with setsockopt().

SO_DEBUG

enables recording of debugging information

SO_REUSEADDR

enables local address reuse

SO_REUSEPORT

enables duplicate address and port bindings

SO_REUSEPORT_LB

enables duplicate address and port bindings with load
balancing

SO_KEEPALIVE

enables keep connections alive

SO_DONTROUTE

enables routing bypass for outgoing messages

SO_LINGER

linger on close if data present

SO_BROADCAST

enables permission to transmit broadcast messages

SO_OOBINLINE

enables reception of out-of-band data in band

SO_SNDBUF

set buffer size for output

SO_RCVBUF

set buffer size for input

SO_SNDLOWAT

set minimum count for output

SO_RCVLOWAT

set minimum count for input

SO_SNDTIMEO

set timeout value for output

SO_RCVTIMEO

set timeout value for input

SO_ACCEPTFILTER

set accept filter on listening socket

SO_NOSIGPIPE

controls generation of SIGPIPE for the socket

SO_TIMESTAMP

enables reception of a timestamp with datagrams

SO_BINTIME

enables reception of a timestamp with datagrams

SO_ACCEPTCONN

get listening status of the socket (get only)

SO_DOMAIN

get the domain of the socket (get only)

SO_TYPE

get the type of the socket (get only)

SO_PROTOCOL

get the protocol number for the socket (get only)

SO_PROTOTYPE

SunOS alias for the Linux SO_PROTOCOL (get only)

SO_ERROR

get and clear error on the socket (get only)

SO_SETFIB

set the associated FIB (routing table) for the socket
(set only)

The following
options are recognized in FreeBSD:

SO_LABEL

get MAC label of the socket (get only)

SO_PEERLABEL

get socket’s peer’s MAC label (get only)

SO_LISTENQLIMIT

get backlog limit of the socket (get only)

SO_LISTENQLEN

get complete queue length of the socket (get only)

SO_LISTENINCQLEN

get incomplete queue length of the socket (get only)

SO_USER_COOKIE

set the ’so_user_cookie’ value for the
socket (uint32_t, set only)

SO_TS_CLOCK

set specific format of timestamp returned by
SO_TIMESTAMP

SO_MAX_PACING_RATE

set the maximum transmit rate in bytes per second for
the socket

SO_DEBUG enables
debugging in the underlying protocol modules.

SO_REUSEADDR
indicates that the rules used in validating addresses
supplied in a bind(2) system call should allow reuse of
local addresses.

SO_REUSEPORT
allows completely duplicate bindings by multiple processes
if they all set SO_REUSEPORT before binding the port. This
option permits multiple instances of a program to each
receive UDP/IP multicast or broadcast datagrams destined for
the bound port.

SO_REUSEPORT_LB
allows completely duplicate bindings by multiple processes
if they all set SO_REUSEPORT_LB before binding the port.
Incoming TCP and UDP connections are distributed among the
sharing processes based on a hash function of local port
number, foreign IP address and port number. A maximum of 256
processes can share one socket.

SO_KEEPALIVE
enables the periodic transmission of messages on a connected
socket. Should the connected party fail to respond to these
messages, the connection is considered broken and processes
using the socket are notified via a SIGPIPE signal when
attempting to send data.

SO_DONTROUTE
indicates that outgoing messages should bypass the standard
routing facilities. Instead, messages are directed to the
appropriate network interface according to the network
portion of the destination address.

SO_LINGER
controls the action taken when unsent messages are queued on
socket and a close(2) is performed. If the socket promises
reliable delivery of data and SO_LINGER is set, the system
will block the process on the close(2) attempt until it is
able to transmit the data or until it decides it is unable
to deliver the information (a timeout period, termed the
linger interval, is specified in seconds in the
setsockopt() system call when SO_LINGER is
requested). If SO_LINGER is disabled and a close(2) is
issued, the system will process the close in a manner that
allows the process to continue as quickly as possible.

The option
SO_BROADCAST requests permission to send broadcast datagrams
on the socket. Broadcast was a privileged operation in
earlier versions of the system.

With protocols
that support out-of-band data, the SO_OOBINLINE option
requests that out-of-band data be placed in the normal data
input queue as received; it will then be accessible with
recv(2) or read(2) calls without the MSG_OOB flag. Some
protocols always behave as if this option is set.

SO_SNDBUF and
SO_RCVBUF are options to adjust the normal buffer sizes
allocated for output and input buffers, respectively. The
buffer size may be increased for high-volume connections, or
may be decreased to limit the possible backlog of incoming
data. The system places an absolute maximum on these values,
which is accessible through the sysctl(3) MIB variable
’’kern.ipc.maxsockbuf’’.

SO_SNDLOWAT is
an option to set the minimum count for output operations.
Most output operations process all of the data supplied by
the call, delivering data to the protocol for transmission
and blocking as necessary for flow control. Nonblocking
output operations will process as much data as permitted
subject to flow control without blocking, but will process
no data if flow control does not allow the smaller of the
low water mark value or the entire request to be processed.
A select(2) operation testing the ability to write to a
socket will return true only if the low water mark amount
could be processed. The default value for SO_SNDLOWAT is set
to a convenient size for network efficiency, often 1024.

SO_RCVLOWAT is
an option to set the minimum count for input operations. In
general, receive calls will block until any (non-zero)
amount of data is received, then return with the smaller of
the amount available or the amount requested. The default
value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a
larger value, blocking receive calls normally wait until
they have received the smaller of the low water mark value
or the requested amount. Receive calls may still return less
than the low water mark if an error occurs, a signal is
caught, or the type of data next in the receive queue is
different from that which was returned.

SO_SNDTIMEO is
an option to set a timeout value for output operations. It
accepts a struct timeval argument with the number of
seconds and microseconds used to limit waits for output
operations to complete. If a send operation has blocked for
this much time, it returns with a partial count or with the
error EWOULDBLOCK if no data were sent. In the current
implementation, this timer is restarted each time additional
data are delivered to the protocol, implying that the limit
applies to output portions ranging in size from the low
water mark to the high water mark for output.

SO_RCVTIMEO is
an option to set a timeout value for input operations. It
accepts a struct timeval argument with the number of
seconds and microseconds used to limit waits for input
operations to complete. In the current implementation, this
timer is restarted each time additional data are received by
the protocol, and thus the limit is in effect an inactivity
timer. If a receive operation has been blocked for this much
time without receiving additional data, it returns with a
short count or with the error EWOULDBLOCK if no data were
received.

SO_SETFIB can be
used to over-ride the default FIB (routing table) for the
given socket. The value must be from 0 to one less than the
number returned from the sysctl net.fibs.

SO_USER_COOKIE
can be used to set the uint32_t so_user_cookie field in the
socket. The value is an uint32_t, and can be used in the
kernel code that manipulates traffic related to the socket.
The default value for the field is 0. As an example, the
value can be used as the skipto target or pipe number in
ipfw/dummynet.

SO_ACCEPTFILTER
places an accept_filter(9) on the socket, which will filter
incoming connections on a listening stream socket before
being presented for accept(2). Once more, listen(2) must be
called on the socket before trying to install the filter on
it, or else the setsockopt() system call will
fail.

struct
accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};

The
optval argument should point to a struct
accept_filter_arg that will select and configure the
accept_filter(9). The af_name argument should be
filled with the name of the accept filter that the
application wishes to place on the listening socket. The
optional argument af_arg can be passed to the accept
filter specified by af_name to provide additional
configuration options at attach time. Passing in an
optval of NULL will remove the filter.

The SO_NOSIGPIPE
option controls generation of the SIGPIPE signal normally
sent when writing to a connected socket where the other end
has been closed returns with the error EPIPE.

If the
SO_TIMESTAMP or SO_BINTIME option is enabled on a SOCK_DGRAM
socket, the recvmsg(2) call will return a timestamp
corresponding to when the datagram was received. The
msg_control field in the msghdr structure
points to a buffer that contains a cmsghdr structure
followed by a struct timeval for SO_TIMESTAMP and
struct bintime for SO_BINTIME. The cmsghdr
fields have the following values for TIMESTAMP by
default:

Additional
timestamp types are available by following SO_TIMESTAMP with
SO_TS_CLOCK, which requests a specific timestamp format to
be returned instead of SCM_TIMESTAMP when SO_TIMESTAMP is
enabled. These SO_TS_CLOCK values are recognized in
FreeBSD:

SO_TS_REALTIME_MICROrealtime
(SCM_TIMESTAMP, struct timeval), default

SO_TS_BINTIME

realtime (SCM_BINTIME, struct bintime)

SO_TS_REALTIME

realtime (SCM_REALTIME, struct timespec)

SO_TS_MONOTONICmonotonic time
(SCM_MONOTONIC, struct timespec)

SO_ACCEPTCONN,
SO_TYPE, SO_PROTOCOL (and its alias SO_PROTOTYPE) and
SO_ERROR are options used only with getsockopt().
SO_ACCEPTCONN returns whether the socket is currently
accepting connections, that is, whether or not the listen(2)
system call was invoked on the socket. SO_TYPE returns the
type of the socket, such as SOCK_STREAM; it is useful for
servers that inherit sockets on startup. SO_PROTOCOL returns
the protocol number for the socket, for AF_INET and AF_INET6
address families. SO_ERROR returns any pending error on the
socket and clears the error status. It may be used to check
for asynchronous errors on connected datagram sockets or for
other asynchronous errors.

Finally,
SO_LABEL returns the MAC label of the socket. SO_PEERLABEL
returns the MAC label of the socket’s peer. Note that
your kernel must be compiled with MAC support. See mac(3)
for more information. SO_LISTENQLIMIT returns the maximal
number of queued connections, as set by listen(2).
SO_LISTENQLEN returns the number of unaccepted complete
connections. SO_LISTENINCQLEN returns the number of
unaccepted incomplete connections.

SO_MAX_PACING_RATE
instruct the socket and underlying network adapter layers to
limit the transfer rate to the given unsigned 32-bit value
in bytes per second.

RETURN VALUES

Upon successful completion, the
value 0 is returned; otherwise the value −1
is returned and the global variable errno is set to
indicate the error.

ERRORS

The call succeeds unless:

[EBADF]

The argument
s is not a valid descriptor.

[ENOTSOCK]

The argument
s is a file, not a socket.

[ENOPROTOOPT]

The option is
unknown at the level indicated.

[EFAULT]

The address
pointed to by optval is not in a valid part of the
process address space. For getsockopt(), this error
may also be returned if optlen is not in a valid part
of the process address space.